Photo-Induced Hydrogen-Atom Transfer

It is well-known that many organic excited states (e.g. the triplet state of benzophenone) can effectively abstract hydrogen atoms from organic compounds such as alkanes and alcohols. This behaviour is not commonly found for metal-containing compounds - a notable exception being the lowest excited state of uranyl ion which abstracts H atoms from alcohols, sugars etc., with the resultant formation of free radicals and U(V) compounds. Recent work has shown that it is very effective in inducing strand breaks in DNA (see Sect. 8). 

---

A review of the phototochemistry of A-oxides has appeared and this includes a section on the photo-induced deoxygenation of heterocyclic A-oxides. A second review deals with the mechanistic aspects of the photochemistry of bicyclic azoalkanes and considers their photo-reduction by hydrogen donors, a process for which hydrogen-atom transfer and CT mechanisms have been suggested. 

The investigations of Bolland and Cooper and Wells, - discussed in Section IV, have been particularly valuable for indicating certain mechanisms for the photosensitized oxidation of cellulosic materials. The initial step is abstraction of a hydrogen atom by the excited dye, instead of any kind of electron-transfer process. Such a mechanism adequately explains the photo-oxidation which proceeds in the absence of moisture. When moisture is present, there is another process superimposed which also leads to photo-oxidation. Processes such as were described in relation to the sensitized photodecomposition of water (see Section III) may be intimately connected with the initiation step of the moisture-induced reaction. 

---